This invention relates to photocathodes and more particularly to a method for expeditiously forming a high sensitivity sodium-potassium-cesium-antimony photocathode.
The sodium-potassium-cesium-antimony photocathode, or S-20 photocathode, is well known in the art. For example, the details of a typical S-20 photocathode process are disclosed by A. H. Sommer in Photoemissive Materials, John Wiley and Sons, Inc., New York, 1968, pages 114-124. As described by Sommer, the formation of an S-20 photocathode requires a complex sequence of processing steps including as many as 50 or more antimony-potassium alternations as well as a large number ofdf antimony-cesium alternations which are time consuming and thus costly.
A modification of this process is described in U.S. Pat. No. 2,770,561 issued to A. H. Sommer on Nov. 13, 1956 and entitled, "Photoelectric Cathode and Method of Producing Same." The S-20 photocathode disclosed in the Sommer patent differs from the S-20 photocathode disclosed in the latter published Sommer article in that the patented photocathode process initially deposits sodium metal on an antimony film until peak photosensitivity is achieved. While the Sommer's patent also eliminates the antimony-potassium and the antimony-cesium alternations, thus providing a faster process than that described in the Sommer article, the resulting photocathode is disclosed to have a sensitivity of approximately 100 microamperes per lumen and thus unacceptable low in sensitivity for most photomultiplier tube applications.
Increases in S-20 photocathode sensitivity were reported by F. R. Hughes in U.S. Pat. No. 3,372,967 issued on Mar. 12, 1968 and entitled, "Method of Making a Multi-Alkali Cathode" and by F. A. Helvy in U.S. Pat. No. 3,658,400 issued on Apr. 25, 1972 and entitled, "Method of Making a Multialkali Photocathode With Improved Sensitivity to Infrared Light and a Photocathode Made Thereby".
The Hughes patent discloses that a combined source of potassium and sodium may be contained within a single channel so that potassium and sodium may be co-evaporated from the source onto a substrate having an antimony layer thereon. A limitation of the Hughes patent includes the difficulty in achieving high reproducibility of photocathode sensitivity due to the wide variation allowed for the amount of sodium tungstate compared to the amount of potassium chromate. A further limitation of the Hughes patent is the generally low photocathode sensitivity reported (about 140 microamperes per lumen).
The Helvy patent which discloses the simultaneous evaporation of sodium and antimony followed by the simultaneous evaporation of potassium and antimony provides a highly reproducible photocathode having an extended red response greater than the red response achieved by using either the Sommer or the Hughes process; nevertheless, the Helvy process requires a highly skilled cathode processing operator to properly perform the simultaneous evaporation steps.
In each of the above-described references sodium is evaporated either to a peak sensitivity or an excess amount of sodium is provided so that the sensitivity reaches a peak and decreases to less than the peak value. It is generally believed that the chemical composition of an S-20 photocathode having optimum sensitivity requires a two to one ratio of sodium to potassium so that the photocathode has a stoichiometric formula (Cs)Na.sub.2 KSb (see Sommer, Photoemissive Materials, pages 114-124).